Zeolite catalyzed fast pyrolysis offers a simple and robust approach to convert lignocellulosic biomass to aromatic hydrocarbons. During catalytic fast pyrolysis (CFP), cellulose, hemicellulose, and lignin are first thermally decomposed to bio-oil vapors that are further converted to aromatics in the presence of a ZSM-5 zeolite catalyst. The high temperatures required for CFP also favor coke formation, an undesired byproduct, through condensation of the oxygenated intermediates on ZSM-5’s outer surface and/or secondary reactions inside its micropores.

Introducing mesopores through desilication represents a possible strategy to enhance mass transport and favor aromatic production over undesired coke formation. The effect of desilication on the structure, acidity, and performance of aluminum-rich ZSM-5 was studied and a detailed characterization of the structure obtained. Results indicate that mild desilication conditions do not significantly affect the elemental composition, crystallographic structure, microporosity, or distribution of aluminum atoms in framework and extraframework sites. However, the number of accessible Brà  à ¸nsted acid sites increased considerably (ca. 50%) as a result of the enhanced mesoporosity. Aromatic yields for desilicated samples were found to increase by 17% in the pyrolysis of red oak compared to the parent zeolite.

The interplay of structural parameters under reaction conditions was also investigated with the objective to identify relationships that would facilitate further catalyst design. Here, we studied commercial and laboratory synthesized ZSM-5 zeolites and combined data from ten complementary characterization techniques in an attempt to identify parameters common to high-performance catalysts. Crystallinity and framework aluminum site accessibility were found to be critical to achieve high aromatic yields. These findings enabled us to synthesize a ZSM-5 catalyst with enhanced activity, offering the highest aromatic hydrocarbon yield reported to date.

To clarify how mesoporous and highly crystalline zeolites decrease coke formation, the role of internal micropore diffusion and external mass transfer in the deposition of carbon was studied. Here, we decoupled the contributions of these parameters through the comparison of conventional in-situ experiments (biomass/catalyst physical mixtures) to zeolites with model compounds pre-adsorbed in the porous structure. Experimental results, diffusion measurements, and the calculation of the mass transfer Biot number point to micropore diffusion as the dominant cause of coke formation. Specifically, the presence of defects in the zeolite’s micropores was found to actively contribute to this undesired side reaction. Conversely, external surface barriers appear to play a minimal role in coking.

In addition to catalyst optimization, the stability of ZSM-5 was studied to gain insights into the dynamic phenomena that may alter the catalyst structure under reaction conditions. Our results suggest high-aluminum content zeolites thermally degrade within hours above 600 à  à °C, emphasizing the importance of operating conditions on long-term catalyst performance. Detailed characterization of the thermally treated zeolites indicated that they retained the desired MFI crystallographic structure but displayed significant changes in Brà  à ¸nsted and Lewis acid site densities due to extensive dealumination. Depending on temperature, up to 50% of the aluminum initially present in the zeolite structure was lost to form extra-framework species that restrict the diffusion of reactants and products inside the catalyst particles. These alterations led to a 70% drop in performance for the catalyzed fast pyrolysis of cellulose. Low aluminum content ZSM-5 zeolites were more stable, suggesting a compromise must be found between reaction temperature and catalyst features to achieve high activity and long-term stability.